In the electronics industry interest in Al.sub.x Ga.sub.1-x As has grown to the point that currently R&D efforts related to this material surpass those of all other III/V compounds and alloys. The nearly exact lattice parameter match between GaAs (a.sub.o =5.654) and AlAs (a.sub.o =5.661) allows the growth of lattice matched heterostructures with nearly ideal interference and direct bandgap energies covering an important range for optoelectronic devices from 1.43 to &gt;2.0 eV. This combination of properties has allowed fabrication of such optoelectronic devices as lasers, high radiance infra-red light-emitting diodes (LEDs) for optical communication applications, visible LEDs, integrated optics elements and circuits, and high efficiency solar cells.
A number of techniques used for the growth of other III/V semiconductors have also been attempted for the growth of Al.sub.x Ga.sub.1-x As, including liquid phase (LPE), molecular beam epitaxial growth (MBE), and vapor phase epitaxial growth (VPE). However, VPE is the technique presently used for all large-scale commercial semiconductor growth operations, including Si, GaAs.sub.x P.sub.1-x and, more recently, GaP. The vapor phase process exhibits excellent control of thickness, composition (for GaAs.sub.x P.sub.1-x alloys) and doping level, and is generally superior to LPE in the areas of morphology and defect control. Another advantage of VPE (which would be particularly useful for Al.sub.x Ga.sub.1-x As structures) is the ease with which composition or doping level can be altered by simply changing the flow rate of the appropriate gas or gases during the growth cycle. Effects such as an intentional taper in composition or doping level which are impossible by LPE are thereby routinely achieved in VPE.
Inspired by the many possible advantages of VPE for growth of Al.sub.x Ga.sub.1-x As, workers in the field have attempted to apply VPE techniques previously used for other III/V compounds and alloys to the Al.sub.x Ga.sub.1-x As system. Thus, chloride transport of Al and Ga in a VPE system has been attempted, but these efforts have been reported as unsuccessful for a variety of reasons, some of which are: (1) Substrate temperatures of over 1000.degree. C. were necessary to grow single crystalline layers of AlAs on GaAs substrates; (2) Even at these high temperatures, predeposition of AlAs (i.e., deposition on the crucible walls upstream from the substrate) hindered deposition on the substrate. Efforts to avoid predeposition resulted in insufficient mixing of the gases prior to deposition; (3) The resulting epitaxial layers were found to be pure AlAs substrate temperatures of &gt;1000.degree. C. and pure GaAs for lower substrate temperatures; (4) AlCl was found to severely attack the fused SiO.sub.2 typically used for reactor walls, thus destroying the apparatus and causing high Si doping in the epilayers; and (5) Oxidation of the metallic Al, with which HCl reacts to form AlCl, was found to totally prevent growth unless many Al boats were arranged in series to allow oxygen to be removed from the gas stream by the earlier boats to prevent an oxide skin from forming on the final boats in the series. Because of these problems no successful growth of Al.sub.x Ga.sub.1-x As alloys by chloride transport has been reported to date.
Another approach which has been somewhat more successful has been the growth of Al.sub.x Ga.sub.1-x As using volatile organometallic Al and Ga compounds to transport the metals into the reaction zone of a cold-wall reactor. Very little work has been reported in this area, but early results indicate that lower temperature growth is possible with excellent control of both alloy composition and morphology. However, Al.sub.x Ga.sub.1-x As grown in this manner is highly contaminated by carbon which acts to compensate the material (i.e., the ratio (N.sub.D +N.sub.A)/(N.sub.D -N.sub.A) becomes large) resulting in high resistivity. More importantly, such material exhibits extremely low photoluminescence (PL) intensity, presumably due to the introduction of non-radiative recombination centers.
No attempts to use organometallics for growth of III/V materials in hot-wall systems have been reported. The known predelection of metal alkyls, especially aluminum alkyls, to decompose homogeneously into carbides and metal-alkyl polymers, as widely reported in the literature on metallization, has discouraged this approach. Experiments performed by the present inventors in a hot-wall system using AsH.sub.3 and organometallics of gallium and aluminum have indeed shown extensive formation of Al.sub.4 C.sub.3 in the inlet regions, along with formation of a fine solid powder. No epitaxial growth was achieved.
Other work of some relevance to the present invention was reported by K. Kindeke, W. Sack, and J. J. Nick in J. Electrochem. Soc. 117, (Oct. 1970) and Y. Nakayama, S. Ohkawa, H. Hashimoto, and H. Ishikawa in J. Electrochem. Soc. 123, 1227 (1976) in which an organometallic halide (Ga(C.sub.2 H.sub.5).sub.2 Cl) was introduced to grow GaAs by a cold-wall VPE deposition process. H. Manasevit and W. Simpson reported in J. Electrochem. Soc. 116, 1725 (Dec. 1969) an unsuccessful attempt to grow GaAs in a cold-wall system by introducing trimethylgallium (TMG) and gaseous AsCl.sub.3. In none of this work was there any suggestion of introducing gaseous halogens or hydrogen halides (as will be discussed below) into the systems which were investigated, nor did any of this work apply to the Al.sub.x Ga.sub.1-x As system in particular.